1. Field of the Invention
The present invention relates to a hydraulic control of a construction machinery operated by hydraulic transmission such as a hydraulic shovel, specifically to a hydraulic control system drastically enhancing the system efficiency improving operating performance thereof.
2. Description of Related Art
There are a negative feedback control system (hereinafter called as “Nega-con system”), a positive feedback control system (hereinafter called as “Posi-con system”), and a load sensing system, etc. as the hydraulic control system of the hydraulic shovel.
The matters in question of the above each conventional system are explained below.
Nega-con System:
FIG. 14 illustrates a hydraulic circuit diagram of the Nega-con system.
In the system, there is provided with a pressure generating means at an outlet of a center bypass passage in a directional control valve with a center bypass passage. The pressure generating means causes the pressure at its upstream to operate as a pressure signal on a control mechanism of pump discharge rate. The control mechanism of pump discharge rate operates to increase the discharge rate in accordance with lowering of the pressure signal.
The system provides preferred operating performance to the hydraulic shovel which an operator handles and therefore is in heavy usage, because a signal fed back to the pump from the directional control valve is low pressure, and pressure oil is supplied to an actuator bleeding-off a part of the pressure oil discharged from the pump. In the case, however the pressure oil supplied to the directional control valve from the pump is cast away in the form of heat as a part of the oil is discharged through the center bypass passage to a tank while the directional control valve is in neutral position or on the way of operation. Particularly in case that a hydraulic actuator connected to the valve is in the type of heavy load, that is, requires large volume of pressure oil to actually start to move and therefore the operation quantity of the directional control valve is large, there is still a problem that the pressure oil is cast away during the time interval and as the result energy is cast away uselessly, though the discharge rate of the pump is held to be small by the pressure oil.
On the other hand it is possible to relatively drop the pressure oil quantity discharged from the center bypass, for instance, by narrowing the center bypass passage of the directional control valve. However, it is very difficult to appropriately adjust areas and timing between the opening of the center bypass passage and the aperture passage to the cylinder port in the directional control valve.
Furthermore, depending on machining accuracy and valve operation, pressure oil discharged from the pump is trapped in a passage of supply lines to cause abnormally high pressure generation that rather deteriorates energy efficiency.
In the hydraulic control system used for the hydraulic shovel, the operating performance under the condition that a plurality of actuators are operated at the same time, that is, characteristics of flow distribution to each hydraulic actuator from the variable displacement hydraulic pump is very important. In general, this characteristics is uniquely defined by aperture areas in each valve on discharge flow rate of the pump at prescribed revolution numbers. Accordingly, there is still a problem that preferred operating performance may not be maintained when revolution numbers of an engine or load pressure change.
Posi-con System:
FIG. 15 illustrates a hydraulic circuit diagram for the Posi-con system.
In the system, pressure oil discharged from a variable displacement hydraulic pump with a control mechanism of the pump discharge rate is supplied to directional control valves with a center bypass passage. The discharge rate of the pump is controlled by operation signals to the valves in such a way that the discharge flow rate of the pump increases in accordance with increase of operation quantity to the valves.
In the Posi-con system, there is a similar problem as in the Nega-con system, in view of system efficiency and operating performance in combined operations in spite of the preferred operating performance, because a part of the pressure oil is discharged through the center bypass passage to a tank while the valve is in neutral position or on the way of operation.
In particular, the discharge rate is uniquely defined in accordance with the operating signals to the valves regardless of opening degree at the center bypass passage. Therefore it is very difficult to adjust areas and timing between the opening of the center bypass passage and the aperture passage to the cylinder port, so there is still a problem that abnormally high pressure generation and delay of rising in pressure may cause in the passage of supply lines from the pump.
In addition, it can be said that the Posi-con system is preferred in response compared with other systems, because the discharge rate of the variable displacement hydraulic pump is controlled by a signal output in parallel with operating signals to the directional control valves.
Load Sensing System
FIG. 16 illustrates a hydraulic circuit diagram of the load sensing system. In the system, as to the oil quantity for supply to the hydraulic actuator, the discharge rate of the variable displacement pump is controlled so that differential pressure at meter-in side of the directional control valve concerned is constant. Accordingly, there is no pressure oil throttled and cast away as in the Nega-con system and the Posi-con system, and system efficiency is improved by the extent.
However, the differential pressure at meter-in side must be maintained on some level, for instance, more than 20 bars in order to control the pump discharge rate appropriately, and as the result, pressure drop occurs in the pressure oil when it passes through each the valve regardless of the quantity. Thus, there is still a problem in view of system efficiency. Furthermore, as to the control of the pump discharge rate, there is another problem of delay in response, because first, the directional control valve is operated, and then, as the result, the pump discharge rate is controlled.